Intracardiac measurements with the impedance technique

The author describes how intracardiac volume can be assessed by means of the impedance presented to the passage of an electric current by the blood contained in that volume. It begins with an overview of impedance measurement, and then describes the measurement of cardiac impedance using transventricular electrodes. The calibration of intraventricular blood conductance in absolute volume terms, a major problem that is not as yet fully solved, is discussed. Three applications are examined: intraventricular pressure-volume diagrams: measurements of cardiac efficiency; and detection of regional myocardial ischemia.